Fumonisins (mycotoxin) and optic neuropathy in cattle

Dr. Steve Ensley

Background: A Canadian beef herd reported a sudden onset of blindness after grazing on standing corn in mid-winter. Fusarium species were isolated in the corn that was being grazed. Blindness was thought to be due to an optic nerve degeneration suspected to be secondary to consuming fumonisin mycotoxins.

History: In a herd of 220 beef cows, 184 mature pregnant cows had been placed on a field containing standing corn. Six days after being placed on this pasture 1 cow was found to be acting blind. On day 9 the producer noted gait abnormalities in 35 cows.

Clinical signs: Affected cows had stiffness in the hindquarters, walking as if they had dislocated hips or walking in a slow motion. Between days 12 and 15 after being placed on the corn ground an additional 4 cows were identified as blind. All cows were removed from the pasture on day 15. The gait abnormalities resolved without therapy within 2 weeks of cows being removed from the corn pasture. Calving began 2 days after the animals were removed from the corn ground. Over the following 3 weeks, cows that were blind calved normally and had viable calves. Approximately 20 calves died shortly after birth or were stillborn. This was slightly higher than normal. There was minimal investigation into the cause of the dead or stillborn calves.

Diagnostics: On day 15 blood samples were collected from 9 cows, 7 with gait abnormalities and 2 that were blind, and submitted for lead analysis. The lead analysis was normal. On day 21, blood from 3 blind cows were submitted for vitamin A and E, biochemistry and mineral analysis. Serum vitamin A values were considered marginal. None of the serum chemistries were outside the normal reference range for adult beef cattle. Five blind cows and 2 calves were examined by a veterinary ophthalmologist. Neuroophthalmic examination of the cows showed absent dazzle reflexes, absent menace responses bilaterally, and widely dilated and fixed pupils with no direct or consensual pupillary light reflexes. Intraocular pressures were estimated with a rebound tonometer (Tonvet; Tiolat, Helsinki, Finland) and ranged from 16 to 26 mmHg. Although rebound tonometry has not been verified in cattle, this range of pressures is within normal limits for other methods of tonometry. Clinical and ophthalmic examination (including a maze test) of the calves of 2 blind cows showed no abnormalities and these calves were determined to be visual. Euthanasia was performed on one of the affected cows. Light microscopic examination of the eyes, optic nerve, and chiasm of this cow revealed spongiosis (dilated spaces), extensive axonal degeneration, and presumed histiocytic infiltration of the optic nerve and optic chiasm.

Treatment: Animals were removed from the corn stubble field where the suspected fumonisins were detected. No other treatment was initiated.

Outcome: All blind cattle recovered their sight. The increased incidence of dead calves at birth was thought to be due to the fumonisin exposure.

Take home message: Monitor for any clinical signs consistent with blindness in cattle on rations containing elevated fumonisins. There have been reports of elevated concentrations of fumonisins in the 2017 corn crop.